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https://issues.apache.org/jira/browse/ARROW-16184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17522834#comment-17522834
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Raphael Taylor-Davies commented on ARROW-16184:
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Do you know if this convention is documented anywhere, this would be a breaking
change to the arrow-rs implementation and so it would be good to have something
authoritative to reference as justification. That being said it seems odd to me
that the less expressive schema would be treated as the authoritative one - if
you can't trust the arrow schema, what is the point in embedding it?
> [Python] Incorrect Timestamp Unit in Embedded Arrow Schema Within Parquet
> -------------------------------------------------------------------------
>
> Key: ARROW-16184
> URL: https://issues.apache.org/jira/browse/ARROW-16184
> Project: Apache Arrow
> Issue Type: Bug
> Components: Python
> Reporter: Raphael Taylor-Davies
> Priority: Minor
>
> As pointed out in https://issues.apache.org/jira/browse/ARROW-2429 the
> following code results in the schema changing when reading/writing a parquet
> file.
> {code:python}
> #!/usr/bin/env python
> import pyarrow as pa
> import pyarrow.parquet as pq
> import pandas as pd
> # create DataFrame with a datetime column
> df = pd.DataFrame({'created': ['2018-04-04T10:14:14Z']})
> df['created'] = pd.to_datetime(df['created'])
> # create Arrow table from DataFrame
> table = pa.Table.from_pandas(df, preserve_index=False)
> # write the table as a parquet file, then read it back again
> pq.write_table(table, 'foo.parquet')
> table2 = pq.read_table('foo.parquet')
> print(table.schema[0]) # pyarrow.Field<created: timestamp[ns]> (nanosecond
> units)
> print(table2.schema[0]) # pyarrow.Field<created: timestamp[us]> (microsecond
> units)
> {code}
> This was closed as a limitation of the parquet 1.x format for representing
> nanosecond timestamps. This is fine, however, the arrow schema embedded
> within the parquet metadata still lists the data as being a nanosecond array.
> This causes issues depending on which schema the reader opts to "trust".
> This was discovered as part of the investigation into a bug report on the
> arrow-rs parquet implementation -
> [https://github.com/apache/arrow-rs/issues/1459]
> Specifically the metadata written is
> {code:java}
> Schema {
> endianness: Little,
> fields: Some(
> [
> Field {
> name: Some(
> "created",
> ),
> nullable: true,
> type_type: Timestamp,
> type_: Timestamp {
> unit: NANOSECOND,
> timezone: Some(
> "UTC",
> ),
> },
> dictionary: None,
> children: Some(
> [],
> ),
> custom_metadata: None,
> },
> ],
> ),
> custom_metadata: Some(
> [
> KeyValue {
> key: Some(
> "pandas",
> ),
> value: Some(
> "{\"index_columns\": [], \"column_indexes\": [],
> \"columns\": [{\"name\": \"created\", \"field_name\": \"created\",
> \"pandas_type\": \"datetimetz\", \"numpy_type\": \"datetime64[ns]\",
> \"metadata\": {\"timezone\": \"UTC\"}}], \"creator\": {\"library\":
> \"pyarrow\", \"version\": \"6.0.1\"}, \"pandas_version\": \"1.4.0\"}",
> ),
> },
> ],
> ),
> features: None,
> } {code}
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